With improvements in liquid crystal display (LCD) technology, LCD televisions including LCD panels are becoming increasingly popular. An LCD panel includes a matrix of pixels that are driven with pixel data values to display a desired image.
In attempts to improve display quality of such LCD panels, subframes are often inserted to form pulse-like image data according to the pulse-like LCD technology. An issue with using LCD panels in televisions is that the perceived image quality can suffer as a result of edge blurring. To address this, subframes are inserted to provide luminance similar to that of a CRT (cathode ray tube) television. With one conventional technique, a normally black subframe is often inserted in each frame, as shown in FIG. 1. FIG. 1 shows two adjacent pixels 101 and 102 for respectively receiving gray-scale data A and B and displaying the gray-scale data A and B in a frame time Tf.
FIG. 2 shows a first pulse-like liquid crystal display technology, in which a normally black subframe (a subframe having a gray-scale value of 0) is inserted into the pixels 101 and 102 along with the gray-scale data A and B, if an image doubled frame rate technology is used. The image doubled frame rate technology refers to using a doubled frame rate so that two subframes of data can be provided in each frame. Thus, the pixels 101 and 102 of FIG. 2 respectively display the subframe with the gray-scale data A and B in the front half frame time (½ Tf), and display a black frame in the rear half frame time (½ Tf). According to the eye-tracking model, the conventional black frame inserting method can effectively halve the blurred width (or brightness edge width). However, the conventional black frame inserting method enables the pixel to display the gray-scale data correctly only during one half of the frame time, and to display the normally black frame of gray-scale data of 0 during the other half of the frame time. Thus, the frame luminance is reduced in half, thereby negatively influencing the image displaying effect.
To improve the problem of the halved pixel luminance caused by the black frame insertion technique, a second conventional subframe insertion technique does not influence the equivalent luminance of the frame. As shown in FIG. 3, when the pixels 101 and 102 receive the gray-scale data A and B, the second subframe insertion technique enables the pixel 101 to sequentially display subframes A′ and C and the pixel 102 to sequentially display subframes B′ and D. The average luminance of the pixel 101 for displaying the subframes A′ and C in the frame time Tf is the same as the luminance effect of directly displaying the gray-scale data A throughout the frame time Tf in FIG. 1. The average luminance of the pixel 102 for displaying the subframes B′ and D in the frame time Tf is the same as the luminance effect of directly displaying the gray-scale data B throughout the frame time Tf in FIG. 1.
FIG. 4 shows an example look-up table 40 used in the second subframe insertion technique of FIG. 3 for generating the subframes. As shown in FIGS. 3 and 4, the second subframe insertion technique sequentially displays two subframes having the gray-scale values of 250 and 0 when the pixel receives an original gray-scale value of 150, and two subframes having the gray-scale values of 255 and 0 when the pixel receives an original gray-scale value of 151. In the look-up table 40 of FIG. 4, the original gray-scale value not greater than 151 is mapped to various gray-scale values for the first subframe and mapped to a black value for the second subframe. The gray-scale values of the first and second subframes together provide a synthesized luminance effect that is equal to the luminance corresponding to the original gray-scale value. In addition, the original gray-scale value greater than 152, is mapped to a gray-scale value of 255 for the first subframe, and mapped to various gray-scale values for the second subframe. The gray-scale values for the second subframe are adjusted to provide a synthesized luminance effect that is equal to the luminance of the original gray-scale value.
In typical image data, the gray-scale values of the adjacent pixels are very close to each other. Thus, if the original gray-scale values of the pixels 101 and 102 of FIG. 3 are both smaller than 151, the gray-scale values C and D of the subframe are equal to 0. If the original gray-scale values of the pixels 101 and 102 are both greater than 152, the gray-scale values A′ and B′ of the subframe are equal to 255. The two conditions can effectively halve the blurred width of the motion picture image without influencing the image displaying luminance.
FIG. 5 is a graph for mapping first and second subframe gray-scale values to original gray-scale values, according to the look-up table 40 of FIG. 4. According to FIG. 5, the gray-scale value of the first subframe is 255 when the original gray-scale value is greater than g51, and the gray-scale value of the second subframe is 0 when the original gray-scale value is smaller than g51. The value of g51 of FIG. 5 may be any reasonable design value. For example, the value of g51 may be 151 for an 8-bit gray-scale display system.
An LCD panel is limited by the response speed of liquid crystal cells. When the gray-scale value displayed by a pixel is changed, the corresponding liquid crystal cell requires a certain response time to reach the target gray-scale value. In some cases, an over-drive technique is used to enable the pixel to switch between low and high gray-scale levels.
FIG. 6 shows a graph illustrating application of the second subframe insertion technique in conjunction with an over-drive technique. The example of FIG. 6 is for an 8-bit gray-scale display system, which has a gray-scale display range from 0 to 255. The pixel sequentially receives the pixel data of four frames f61, f62, f63 and f64 in time periods from t61 to t63, from t63 to t65, from t65 to t67 and from t67 to t69, respectively. The original gray-scale values of the four frames are successively 32, 32, 64 and 64. Thus, the liquid crystal cell sequentially receives the control voltages of V(L2), V(L0), V(L2), V(L0), V(L4), V(L0), V(L3) and V(L0) provided to the pixel according to the second subframe insertion technique. The corresponding luminances of the pixel are represented as L2, L0, L2, L0, L3, L1, L3 and L1, respectively. Note that the luminances are represented as triangular waves where increases and decreases in luminance slope upwardly or downwardly according to response times of the corresponding liquid crystal cell. However, if the response speed of the liquid crystal cell is not high enough, the liquid crystal cell cannot be charged to the voltage value for correctly displaying the gray-scale luminance L3 (for frame f63) if the liquid crystal cell is directly driven by the pixel control voltage V(L3) corresponding to the gray-scale luminance L3 after the gray-scale luminance L0 (in the previous frame f62). Thus, as shown in FIG. 6, an over-drive voltage is applied to drive the liquid crystal cell in frame f63. That is, a new pixel data voltage higher than the original pixel control voltage is applied to the liquid crystal cell from the time instant t65 to the time instant t66. For example, the control voltage V(L4) corresponding to the gray-scale luminance L4 (L4>L3) of FIG. 6 is applied so that the pixel can display the gray-scale luminance L3 immediately and correctly. Similarly, if the response speed of the liquid crystal cell is not high enough, the pixel still can only display the gray-scale luminance L1 rather than the full black at the time instant t67 although the control voltage is dropped to 0 from the time instant t66 to the time instant t67. Because the pixel is not fully black at the time instant t67, no over-drive voltage has to be applied from the time instant t67 to the time instant t68, and only the control voltage V(L3) correctly corresponding to the gray-scale luminance L3 needs to be applied for the pixel to correctly display the gray-scale luminance L3.
However, the conventional pulse-like liquid crystal display adopting the driving technique of FIG. 6 usually has the problems of double-boundary (or double image) and poor MPRT (Motion Picture Response Time), which degrades motion picture quality. For example, the double-boundary problem results from the integration areas of the frame times between t63 and t65 and between t65 and t67 being significantly different from each other.
FIG. 7 shows an eye stimuli integration curve corresponding to the technique of FIG. 6, wherein the horizontal axis represents the time, the vertical axis represents the normalized intensity, and the turning portion of A is where the double-boundary occurs. Thus, although the driving technique of FIG. 6 can be used for the purpose of correcting the image by re-adjusting the single subframe data of a single frame, the technique cannot improve the double-boundary problem completely, and even induces the condition of boundary overshooting or boundary undershooting.
In addition, an NBET parameter is widely used to represent the motion picture quality. The NBET parameter is defined as follows:NBEW=BEW/velocity,  (Eq. 1)NBET=NBEW/frame rate,  (Eq. 2)where BEW is the blurred boundary width of the motion picture image. A smaller NBET value represents less blurred boundary of the motion picture image and thus better motion picture quality. A greater NBET value is obtained when the phenomenon illustrated by the turning portion of A in FIG. 7 occurs, increasing the blurred boundary and decreasing the motion picture quality.